Cleavage of diacrylic acid to form acrylic acid



April 6, 1963 N. VON KUTEPOW ETAL 3,085,046

CLEAVAGE OF DIACRYL-IC. ACID TOIFORM ACRYLIC ACID Filed Jan. 21, 1960 INVENTORS: NIKOLAUS vow KUTEPOW HELMUT DOERFEL ATT'YS United States Patent 3,086,046 CLEAVAGE 0F DIACRYLIC ACID TO FORM ACRYLIC ACID Nikolaus vou Kutepow, Karlsruhe-Ruppurr, and Heimut Doerfel, Ludwigsllafen (Rhine), Germany, assignors to Badische Anilin- & Soda-Fabrik Aktiengesellschaft, Ludwigshafen (Rhine), Germany Filed Jan. 21, 1960, Ser. No. 3,933 Claims priority, application Germany Jan. 30, 1959 4 Claims. (Cl. 260-526) This invention relates to a process for the continuous cleavage of acrylic acid derivatives in the gas phase to form acrylic acid.

In the synthesis of acrylic acid from acetylene, carbon monoxide and water in the presence of suitable catalysts, for example nickel bromide and copper iodide, at increased pressure and elevated temperature, various acrylic acid derivatives are obtained as byproducts in addition to the main product which is acrylic acid. Besides diacrylic acid there are formed polyacrylic acids having the structural features of diacrylic acid, polyacrylic acids connected by carbon-carbon linkages, hydracrylic acid and polyesters of hydracrylic acid. Since no use for the mixture of these acrylic acid derivatives is known, attempts have been made to split up these compounds into acrylic acid.

It is known to cleave the said acrylic acid derivatives in the liquid phase at temperatures of 35 to 85 C. and at pressures of 10 to 100 mm. Hg absolute in the presence of acid catalysts, such as sulfuric acid, phosphoric acid, para-toluene-sulfonic acid and the like, to form acrylic acid. This cleavage in the liquid phase has considerable disadvantages, however, especially because it cannot be carired out continuously.

That fraction of the residue remaining in the cleavage vessel after distilling off the acrylic acid, which does not undergo cleavage and contains the catalyst from the acrylic acid synthesis and certain unidentified dark-colored resinous constituents, is turned in the course of the cleavage into such a viscous reaction mixture that a continuous Withdrawal of the non-cleavable fraction from the cleavage vessel becomes impossible for mechanical reasons. Nor can the reaction mixture be stirred. This leads to incrustation on the walls of the vessel in which the cleavage is carried out and the heat transfer from the vessel wall to the material to be split up is thereby worsened. Cleavage in the liquid phase must therefore be carried out discontinuously. After every batch, the reaction vessel must be cleaned mechanically and by boiling out with soda.

We have now found that in the catalytic production of acrylic acid from acetylene, carbon monoxide and water, the acrylic acid derivatives formed as byproducts can be continuously split up into acrylic acid by vaporizing the acrylic acid derivatives at a pressure of between about 5 and 150, preferably and 100 mm. Hg absolute and leading the vapors under reduced pressure through a vessel heated to 350 to 650 0., preferably to 450 to 600 C.

Surprisingly, when the vapors of acrylic acid derivatives are led through heated vessels no incrustation or formation of carbon black takes place on the walls. It is therefore possible to carry out the thermal cleavage of acrylic acid derivatives in the gas phase continuously.

As vessels in which the thermal cleavage takes place it is preferable to use tubes which have a large wall surface in relation to the reaction space. Tubes of stainless steel with an inside diameter of about to 30 limeters have proved suitable. Only relatively short periods are required for the thermal cleavage. The residence periods in the cleavage tubes are about 0.2 to

2 seconds. The speed of flow of the vapors may be between about 10 and 50 meters per second. The walls of the vessel in which the cleavage is carried out (in gen eral these are stainless steel tubes) are heated to temperatures between 350 to 650 0., preferably between 450 and 600 C. The vapors of the acrylic acid derivatives to be split up have temperatures between about 300 and 500 C.

For example a throughput of 13.5 kilograms of cleavage acrylic acid per day per liter of reaction space can be achieved in a tube of stainless steel having a plurality of bends, a diameter of 20 millimeters and a length of 10 meters with a wall temperature between 600 and 620 C. and a vapor temperature of about 380 C., a vapor speed of 30 meters per second and a residence period of 0.33 second. In a stainless steel tube having a diameter of 70 mm. and a length of 7 meters and with a tube wall temperature of about 580 C., with a steam temperature of about 460 C., a vapor speed of 5.6 meters per second and a residence period of 1 second, the throughput is only 2.42 kilograms of cleavage acrylic acid per liter of reaction space per day. The cleavage reaction takes place at the hot tube wall because under approximately identical conditions the cleavage outputs are proportional to the tube wall surface.

The acrylic acid derivatives to be split up, which in general remain in the sump of the column in the distillation of the crude acrylic acid, are continuously vaporized in a vaporizer at heating temperatures of between and 270 C. and at pressures between 2 and 200 mm. Hg and led from the top of the vaporizer directly into the inlet of the cleavage tube. Turbulent-film Vaporizers, especially Sambay turbulent-film vaporizers, have proved suitable for the purpose. 'Upon leaving the cleavage tube the cleavage products .are cooled as quickly as possible to about 30 to 40 C. and condensed.

According to the process of the present invention it is possible to split up the distillation residues arising in the distillation of acrylic acid obtained catalytically from acetylene, carbon monoxide and water, to the extent of about 60 to 80% by weight into acrylic acid which can be worked up in known manner, possibly after distillation. It is surprising that at the relatively high temperature at which the cleavage of the acrylic acid derivatives is carried out, no formation of carbon black or blockage of the cleavage vessel occurs but the acrylic acid present in free form and in the form of low molecular products in the initial mixture can be substantially recovered.

The following examples will further illustrate this invention but the invention is not restricted to these examples.

Example 1 1,951 kilograms of crude acrylic acid distillation residue (consisting of 1,510 kilograms of diacry-lic acid, 164 kilograms of acrylic acid, 97 kilograms of hydracrylic acid and compounds not identified analytically) are evaporated in the course of 272 hours in a Sambay turbulentfilm vaporizer A (see the accompanying diagrammatic drawing) at a heating temperature of C. and a pressure of 30 to 60 mm. Hg to the extent of 42% by weight. From the top of the vaporizer, the vapors of the acrylic acid derivatives to be split up pass into a stainless steel tube B 20 mm. in diameter and 10 meters in length which is bent a plurality of times and of which the wall is heated to 600 to 620 C. The speed of the vapors, in the cleavage tube is 30 meters per second and the residence period of the vapors is 0.33 second.

The vapors leaving the tube B at a temperature of 380 C. are condensed in a water-cooled condenser C and in a subsequent cold trap D at 20 to 40 mm. Hg.

aoeeoae The cleavage distillate of 824 kilograms consists of 617 kilograms of acrylic acid (74.8%), 159 kilograms of diacrylic acid (19.3%) and 48 kilograms of components not analytically identified. 45 kilograms of acrylic acid, 935 kilograms of diacrylic acid, 56 kilograms of hydracrylic acid and some unidentified components are contained in the non-vaporizable residue from the Sambay vaporizer (11,127 kilograms), and these can be again evaporated and split.

The above-mentioned analytical data show that 477 kilograms of acrylic acid have been produced by thermal gas-phase cleavage of diacrylic acid and hydracrylic acid. The throughput is 1 3.5 kilograms of cleavage acrylic acid per liter of reaction space per day.

Example 2 In 20 hours, 482 kilograms of crude acrylic acid distillation residue (consisting of 79 kilograms of acrylic acid, 333 kilograms of diacrylic acid and components not identified analytically) are evaporated in a Sambay turbulent-film vaporizer A at a heating temperature of 240 C. and a pressure of 38 mm. Hg to the extent of 80% by weight. The vapors of the acrylic acid derivatives to be split are led through an electrically heated tube B of stainless steel having a diameter of 50 mm. and a length of 30 meters. The wall of the cleavage tube has a temperature of 478 C., and the vapors have a temperature of 416 C. The mean vapor speed in the cleavage tube is 47 meters per second, the residence period of the vapors 0.64 second.

After leaving the cleavage tube, the vapors are condensed at 20 mm. Hg in a Water-cooled condenser C and in a trap D cooled with brine. The cleavage distillate consists of 284 kilograms of acrylic acid (76.5%), 46.4 kilograms of diacrylic acid (12.5%) and 401 kilograms of compounds not identified analytically. In the nonvaporizable residue from the turbulent-film vaporizer (83 kilograms) there are contained 11.5 kilograms of acrylic acid, 53.5 kilograms of diacrylic acid or polyacrylic acids having the structural features of diacrylic acid, and some unidentified components.

Example 3 During operation for 18 hours, 473 kilograms of crude acrylic acid distillation residue containing 45.5 kilograms of acrylic acid, 400 kilograms of diacrylic acid and some components not identified analytically are evaporated in a Sambay turbulent-film vaporizer A at a heating temperature of 240 C. and a pressure of 44 mm. Hg to the extent of 83% by' weight. The vapors of the acrylic acid derivatives to be split pass from the head of the vaporizer into an electrically heated stainless steel tube B of mm. diameter and 30 meters length. The temperature of the tube Wall is 538 C. and the temperature of the vapors is 441 C.; the speed of the vapors is 48 meters per second and the residence period of the vapors in the cleavage tube is 0.62 second.

The vapors consisting mainly of acrylic acid, after leaving the cleavage tube, are condensed at 22 mm. Hg in a water-cooled condenser C and in a trap D cooled with brine. They contain, besides 306 kilograms of acrylic acid (79.5%), 35 kilograms of diacrylic acid (9.1%) and 44 kilograms of components not identified analytically. The residue from the turbulent-film vaporizer is kilograms. The yield of acrylic acid, with reference to acrylic acid distillation residue, is 65%.

What we claim is:

1. A process for the production of acrylic acid by continuous cleavage of diacrylic acid, which comprises: leading the vapors of said diacrylic acid at a pressure of between 5 and mm. Hg through a vessel heated to a temperature of about 350 to 650 C.

2. A process for the production of acrylic acid by continuous cleavage of diacrylic acid, which comprises: vaporizing said diacrylic acid at a pressure of between 5 and 150 mm. Hg and a temperature of between 180 and 270 C. and leading the vapors at a pressure of between 5 and 150 mm. Hg through a vessel heated to a temperature of about 350 to 650 C.

3. A process as claimed in claim 2 in which the vapors have a residence period in said vessel of about 0.5 to 2 seconds.

4. A process as claimed in claim 2 in which the speed of flow of the vapors in said vessel is between about 10 and 50 meters per second.

References Cited in the file of this patent UNITED STATES PATENTS 2,435,510 Redmon Oct. 18, 1949 

1. A PROCESS FOR PRODUCTION OF ACRYLIC ACID BY CONTINUOUS CLEAVAGE OF DIACRYLIC ACID, WHICH COMPRISES: LEADING THE VAPORS OF SAID DIACRYLIC ACID AT A PRESSURE OF BETWEEN 5 AND 150 MM. HG THROUGH A VESSEL HEATED TO A TEMPERATURE OF ABOUT 350* TO 650* C. 